This invention is directed to a process and system for forming composite laminate parts utilizing resin transfer molding in combination with a trapped expansion member.
Various molding systems are known for forming composite laminate parts. In each of these a resin having reinforcing fibers located therein is cured to solidify the resin. One of the most widely utilized composite laminate systems is common glass fiber reinforced plastic is prepared by incorporating finely chopped glass fibers or a glass fiber fabric or mat within a body of resin. The resin typically is cured via a catalysis at room temperature and ambient pressure. Fiber loading is normally low and thus strength is limited.
For high performance applications, as for instance aerospace applications, more exotic composite systems and formation methods are utilized. Prepregs of resin impregnated filaments are available from various manufacturers. These prepregs are shaped and cured utilizing a mold. Prior to curing, the prepreg is shaped to the contour of the mold by the application of pressure.
One pressure technique utilized to shape the prepreg to the mold is vacuum forming. In this technique, a prepreg is laid over a mold surface and covered with a vacuum bagging material. Vacuum is then created between the mold and the vacuum bagging material to shape and hold the prepreg to the mold. The vacuum must be maintained during curing of the prepreg resin to maintain it in the shape of the mold. While this system is very utilitarian it has certain disadvantages. One such disadvantage results from having to maintain the vacuum between the vacuum bag and the mold during the curing cycle. Since the curing cycle is typically done in an autoclave, this requires complicated fixtures and the like within the interior of the autoclave for propagating vacuum to the vacuum bag. Further in many instances a bleed cloth is necessary between the prepreg and the vacuum bag to soak up excess resin that bleeds off of the prepreg during consolidation. Typical of this method of fabrication is that described in U.S. Pat. No. 4,683,099, to Buxton, et al.
In order to eliminate the necessity of vacuum bagging, fixed volume trapped rubber molding apparatuses are utilized. In this system a multiple component mold is used. A cavity is located in the mold in the shape of the item to be molded. A bleed cloth is located in the mold followed by a layer or layers of prepreg as necessary for the composite part. A rubber member having a complementary surface to the part to be molded is then inserted in the mold and the mold closed. The mold is heated to cure the resin and simultaneously to expand the rubber member. Expansion of the rubber member consolidates the prepreg and squeezes excess resin into the bleed cloth. Typical of such a system is the system described in U.S. Pat. No. 4,271,116 to Jones.
In a modification of the above system typified by Jones, a foam or other light weight core is incorporated into the composite part by loading it in the mold with the prepreg. Such a structure is shown in U.S. Pat. No. 4,167,430 to Arachi. Further, the basic system typified by Jones can be augmented by modifying the molds to include one part of the mold being movable such that it can be hydraulically pressed against the remainder of the mold. This is done to increase the pressure transmitted to the prepreg located in the interior of the mold. Typical of this is U.S. Pat. No. 4,624,820 to Barraclough.
In a further adaptation of the above systems typified by Jones, duel rubber expansion members are used. A first expansion member defines a surface of the part being formed. The second expansion member is independently heated or cooled for pressurizing the first and the prepreg trapped between the first and a molding surface. Such a system is described in U.S. Pat. No. 4,812,115 to Kemp.
Irrespective of the mold system used, the use of a prepreg has an inherent disadvantage--a prepreg has very limited shelf life. To enable it to polymerize, the resin component of the prepreg must include all its necessary polymeric precursors; however, these precursors must be inhibited from prematurely curing prior to shaping and consolidation. This normally requires refrigerated shipping and storage to extend its shelf life. Further in using a prepreg, as is exemplified in the above referenced Barraclough and Kemp patents, consideration must be given to coordination of the heating and expansion of the rubber member in order to maintain the proper pressure profiles and volumes during consolidation of the prepreg to the composite part. Additionally, since there is little or no gap left between the consolidating rubber member and the prepreg after it is located in the mold, it is sometimes very difficult to close the mold about the prepreg. Difficulty in closing the mold can result in increase labor expense associated with using prepregs.
An older molding technique that was used prior to the introduction of prepreg is referred to as resin transfer molding. It has certain advantages compared to hand lay-up and autoclave curing and other labor and capital intensive processes for high performance environments, such as the above described use of prepreg. While resin transfer molding has certain advantage, it also has a disadvantage. In present resin transfer molding techniques fiber loading is low. Present resin transfer molding techniques have only been able to achieve maximum fiber loading of 50 or 60% by weight.
In resin transfer molding a two sided mold having a controlled gap between the sides of the molds is loaded with a dry fiber preform. These fiber preforms differ from prepregs since they do not contain resin or resin precursors. The various parts of the mold are secured together and resin is injected into the fiber preform within the mold to saturate the fiber preform and shape the resin and fibers to the surfaces on both sides of the part.
While resin transfer molding does not require the use of prepregs it does require the use of expensive two sided molds. Because a two sided mold must be used, with very few exceptions, the molds must be constructed from metals. Depending upon part complexity, the machining and fabrication of these metal molds can be very expensive. In common with the above described prepreg molding techniques, loading of these molds with preform fibers can be time consuming because the volume of the preform fiber, like the volume of the prepreg, approaches that of the finished component part. This complicates the loading. A recent review of current resin transfer molding techniques is: Resin Transfer Molding For Advanced Composites, Debbie Stover, Advanced Composites, March/April 1990, page 60.